In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules either to a donor roller or to a latent image on the photoconductive member. The toner attracted to a donor roller is then deposited on a latent electrostatic images on a charge retentive surface which is usually a photoreceptor. The toner powder image is then transferred from the photoconductive member to a copy substrate. The toner particles are heated to permanently affix the powder image to the copy substrate.
In order to fix or fuse the toner material onto a support member permanently by heat and pressure, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rollers with the toner image contacting the heated fuser roller to thereby effect heating of the toner images within the nip. In a conventional two roll fuser one of the rolls is typically provided with a layer or layers that are deformable by a harder opposing roller when the two rollers are pressure engaged. The length of the nip determines the dwell time or time that the toner particles remain in contact with the surface of the heated roller.
Roller fusers work very well for fusing images at low speeds since the required process conditions such as temperature, pressure, and dwell can easily be achieved. When process speeds approach 100 pages per minute (ppm) roller fusing performance starts to falter. At such higher speeds, dwell must remain constant which necessitates an increase in nip width. Increasing nip width can be accomplished most readily by either increasing the roller rubber thickness and/or the outside diameter of the rollers. Each of these solutions reach their limit at about 100 ppm. Specifically, the rubber thickness and durometer (softness) are limited by the thermal and physical properties of the material. The roller size becomes a critical issue for reasons of space, weight, cost, and stripping.
Belt fusers, such as those disclosed in U.S. Pat. Nos. 5,250,998 and 5,465,146, are a type of toner image fixing device in which an endless belt is looped around a heating roller, a conveyance roller, and a pressure roller. The pressure roller presses a sheet having a toner image onto the heating roller with the endless belt intervening between the pressure roller and the heating roller. The fixing temperature for the toner image is controlled on the basis of the temperature of the heating roller detected by a sensor, such as a sensor in the loop of the belt and in contact with the heating roller. A first nip region is formed on a pressing portion located between the heating roller and the fixing roller. A second nip region is formed between the belt and the fixing roller, continuing from the first nip region but without contacting the heating roller. The disclosures of U.S. Pat. Nos. 5,250,998 and 5,465,146 are incorporated by reference.
Most belt fusers, however, take significantly more space than more conventional roller fusers. Thus, marking machines, such as electrostatographic reproduction machines, incorporating belt fusers must have larger housings, which is undesirable. Therefore, there is a need for more compact belt fusers.
Embodiments comprise a belt fuser with elongated fusing nip and compact overall size, primarily for use in marking machines, such as a reproduction machine. The compact long nip width fusing apparatus includes, in embodiments, a rotatable fuser roller about which a fuser belt is reeved to form the fusing nip, the tension roller being acted upon by an adjustable or constant force mechanism to engage and place tension on the belt. The belt fuser also includes floating idler rollers about which the belt is reeved. The resulting belt fuser has a longer nip and dwell time than roller fusers, better thermal efficiency and lower fusing temperature than roller fusers, but occupies only slightly more space than a conventional roller fuser.